That is the suggestion of a new piece of research published recently in the Journal of Marketing Research.  

The study looked at the behaviour of those participating in the communities at Prosper.com (a lending website) and eBay.

It found that members of these communities had such a strong sense of belonging and security from being a member of these communities that it led them to making some risky financial decisions.

“Emerging evidence indicates that online community participation impacts many aspects of consumer behavior, and our findings reveal that this impact extends to financial decision-making,” says Utpal Dholakia, professor of management at Rice University.

“Participants in these sites somehow come to believe that their fellow community members will come to their aid when something goes wrong, but in reality, they are out there on their own and could suffer adverse consequences.”
 
For instance when they studied the community at Prosper.com, which is a peer to peer lending site similar to Zopa, took greater risks than non-members.  For instance they'd tend to lend their money to people with worse credit ratings and higher chance of default than would normal members of the public.
 
At eBay active community members were found to engage in riskier bidding behaviours.  They were proved to both bid more often and pay more money for the products they bought.
 
What's interesting is that the more active the person in the community, the riskier their behaviour became.  These effects were stronger when the point of the community was not social in nature.
 
“These communities are different from social networking sites like Facebook, because the individuals involved are usually strangers whose identities are unknown to the consumer,” Dholakia says.
 
As community managers we all want to have members that are highly engaged, but this new research casts a warning that all community managers should be aware of, especially in these financially heightened times.